Marine electromagnetic induction studies - Marine EM Laboratory

MARINE E.M. 309
non-planar source-field morphology and static distortion on the MT data are
shown to be minimal by Bahr and Filloux (1989), who demonstrate that estimates
of the first four harmonics of the Sq response are consistent with plane-wave MT
responses at similar frequencies.
In order to obtain more detailed information over the Juan de Fuca Ridge than
was possible during the EMSLAB experiment, the EMRIDGE programme saw the
deployment of 12 instruments to obtain 11 magnetometer sites and 2 E-field sites
within the flanks and depression of the ridge (unpublished EMRIDGE cruise
report, 1988). All of the instruments were recovered in November 1988, and initial
results again indicate that there is no conductivity signature for the ridge (Hamano
et al., 1989), implying the absence of a large, continuous magma chamber. Another
interesting result reported by Hamano et al. (1989) is that useful E-field data may
be obtained within the MT frequency band without employing water choppers or
very long antennae.
The estimation of lithospheric thickness and its probable increase with age has
been one of the goals of marine MT sounding for some time. After Oldenburg
(1981) reported a strong correlation between lithospheric thickness and age, further
work by Oldenburg et al. (1984) showed that although the data demanded different
structure beneath the different sites, the correlation with age was not as strong as
previously thought.
Niblett et al. (1987) operated a MT station on sea ice in the Arctic Ocean for one
month. During that time the ice sheet moved back and forth over the Alpha Ridge,
a topographic high on the Arctic seafloor. Apart from the technical difficulty of
operating MT equipment in subzero temperature, the experiment is novel because
in many respects it is equivalent to a seafloor sounding, except of course that the
measurements were made on the sea surface. The problems of contamination by
water motion and insensitivity to shallow seafloor structure are the same as those
for seafloor measurements. The results were very sensitive to seafloor topography,
and 2D modelling of bathymetry accounted for the anisotropy observed in the data.
It should be noted that seafloor topography up to 300 km from the measurement
site was considered to influence the data. No lateral structure in seafloor conductiv-
ity was required, and the data were fit by a 1000 f~m lithosphere underlain by a
10 f~m mantle at a depth of 85 km. As noted by the authors, this electrical
asthenosphere is shallow for the inferred age of the seaftoor in this region (100 My).
Berdichevsky et al. (1984) used 2D finite difference modelling of a horst type
structure to conclude that seafloor MT and GDA experiments (in contrast to
observations at the sea surface) are relatively insensitive to such topographic
irregularities.
3.3. THE COAST EFFECT
It was stated above that the GDS method is particulary sensitive to lateral
variations in conductivity. The largest variation of this kind is, of course, the
junction between land and sea, or coast, and indeed the shorelines of the world